hear the grass grow

English

(idiomatic) To have an extremely sensitive sense of hearing.
- 1855, Thomas Bulfinch, Bulfinch's Mythology: The Age of Fable, ch. 31 "Northern Mythology",
  Heimdall is the watchman of the gods. . . . So acute is his ear that no sound escapes him, for he can even hear the grass grow and the wool on a sheep's back.
(idiomatic) To hear very well due to being in an exceptionally quiet environment.
- 2005, Tony Francis, "St Neots dreams dashed along the road to Cup final," telegraph.co.uk, 28 Aug. (retrieved 1 Aug. 2008),
  A suburban arena that was so sepulchral you could hear the grass grow.
(idiomatic) To be very aware or discerning; to pay attention to every small detail.
- 1995, Hans-Peter Schwarz, Konrad Adenauer: a German politician and statesman, Berghahn Books, →ISBN, p. 397,
  He was quick to appreciate Blankenhorn's engaging personality and intelligent use of his skills. Chief among these was a remarkable ability to keep track of developments—to ‘hear the grass grow’.

which sense?